Engine light on code reads bad cat replaced it light returns reads ox senors replaced it light returns reads cat gone bad.
the ECU (brain) has no idea at ALL what is broken!
it only knows 2 things, bad inputs. and failed tests.
nothing else. The brain is not AI (artificial intelligence)
Not even and expert system , yet.
that is the job of and ASE mechanic
to find the cause. and not GUE$$.
your failure to state the DTCs reported , sorry, is bad news to anyhelper, sorry!
420s. is , weak CAT test. but in most case it's not the CAT.
senors replaced? 1 or both.? B1S1 and B1S2?
Did you get o2 (oxy) DTC failures?, if yes, ignore all cat fails.!
if other codes/ DTC, that too, means ignore cat failure.
the CAT monitor (P042x) only is valid if the engine runs prefectly.
if not the CAT overheats or underheats( goes dead cold)\
the cat will over heat, (cel flash madly) for gross engine over fueling (1 example , bad fuel reg, and pressure at 60PSI)
the cat glows red hot , or near. and the test may fail.
case 2: the engine (AFR fuel) is grossly lean the CAT may go under temperature and drop off line.
we call this CAT no lit off, it has a minium working temperature.
as air leaks in front of the cat causes EFI to go very rich.
and leaks behind can make the rear 02 go nuts. and lie
and say, bad Cat.
all this is the domain of the ASE mech. he knows all stuff.
and never ever lets the ECU tell him what to do.;
it does tell him (suggests) a battery of test.
sure.
hope that helps,
post all stored, DTC,all pending and the full freeze frame data
and if cagey? the closed loop status hot idle.
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